This invention relates to an apparatus for detecting such abnormalities of wheels of a vehicle as due to a puncture of any of the tires which are caused during running of the vehicle.
As well known in the art, many of the fundamental requirements of a vehicle such as safety comfort and economy depend on performance of the tire. One of factors in managing tire performance is air pressure charged in the tires.
An air pressure sensor is conventionally provided for individual tires to detect the air pressure, and an inner pressure of the tire is directly detected by the air pressure sensor. This air pressure sensor, however, rotates along with the tire. Accordingly, a sensor signal must be sent to a judging circuit through a contactless transmission unit, giving rise to a very complicated and expensive detection apparatus. Disadvantageously, the air pressure sensor is required to be of high durability and reliability because it is used in a bad environment greatly affected by vibrations, shocks, centrifugal force and temperatures.
Methods for indirect detection of the air pressure include a proposal wherein the distance between an axle of each of the four wheels and the ground is measured, and a tire having a reduced distance from its axle is considered to be abnormal in air pressure. Another proposal as disclosed in Japanese Patent Unexamined Publication No. 54-33772 wherein rotation speeds of the four wheels are detected and a tire rotating at a higher rotation speed is determined to be abnormal in air pressure. The former proposal is disadvantageous in that stable judgement is possible only when the vehicle remains stopped, preventing rapid detection of the distance when tire puncture occurs during running of the vehicle. The distance from the ground will also become difficult to measure depending on the topographic geometry (evenness or unevenness) of the ground. The latter proposal utilizes a decrease in tire diameter due to a decreased air pressure to detect rotation speeds of the four wheels, thereby deciding that a tire rotating at a higher rotation speed is abnormal. Accordingly, taking an instance wherein, in respect of the right and left wheels, the rotation speed of the inside wheel becomes different from that of the outside wheel when the vehicle travels along a curved path, for example, the inside wheel rotation speed must be corrected by a steerage angle due to steering. The latter proposal is therefore required to have an additional sensor or the like for detection of steering angles which makes the construction complicated. In addition, the judgement will become difficult to achieve when the tire wears down and its diameter decreases.